The use of dual-voltage vehicle electrical systems in which a first on-board electrical subsystem provides a 42-V voltage for supplying 42-V consumers and a second on-board electrical subsystem provides a 14-V voltage for supplying 14-V consumers is known in the automotive industry. A DC-DC converter which is used, for example, for transforming the 42-V voltage into the 14-V voltage as a step-down transformer is provided between the two on-board electrical subsystems. Such a DC-DC converter has one or more power transistors which are triggered by a control unit as desired.
Furthermore, bringing the engine of a motor vehicle to its operating temperature as quickly as possible by using an additional electric heater after starting the vehicle is known, since internal combustion engines have a considerably higher efficiency and lower emissions at their operating temperature than in the cold state immediately after starting the vehicle.
This additional electric heater is used for heating the cooling water of the motor vehicle. For this purpose, a heating resistor supplied by a generator and provided for the purpose of heating the cooling water may be used.